Dynamic allocation of radio capacity in TDMA system

ABSTRACT

The invention relates to a method aiming at dynamic division of the radio capacity in a TDMA system dynamically between packet radio service and circuit switched service. According to the invention, some basic number of time slots are reserved for packet radio service and the rest are reserved for circuit switched service. When the traffic requirement of packet radio service increases, information regarding this is obtained by means of a request from a mobile station or through traffic measurement at the base transceiver station. This information is used as a criterion in allocating more time slots to packet radio service.

FIELD OF THE INVENTION

The present invention relates to a method by which radio capacity isdivided dynamically between packet radio service and circuit switchedservice in a TDMA system in which two-way traffic between base stationsand mobile stations takes place in time slots on predetermined channels.The invention also relates to a mobile system operating according to themethod.

BACKGROUND OF THE INVENTION

Most present-day cellular networks offer circuit switched services forspeech and data. The available frequency band is divided between theseservices, all having equal priority. The network does not take intoaccount whether the capacity is used for the transmission of speech ordata.

Considering burst-form data services, circuit switching does not exploitthe channel optimally. Therefore packet radio services are usedalongside existing circuit switched services in cellular networks. Sincethe existing radio band cannot be expanded, packet radio services mustbe fitted into the same band as circuit switched services. Thus acertain amount of capacity has to be taken from circuit switchedservices for packet radio services.

In TDMA cellular systems, a radio band is usually divided into multiplefrequency bands (FDMA, Frequency Division Multiple Access), and eachfrequency band is further divided into multiple time slots. Logicalchannels are transmitted in the physical time slots of the radiointerface. In circuit switched cellular systems, all time slots are usedfor control signaling and for circuit switched traffic.

Cases associated with this topic have been discussed previously incertain patent publications. One example of them is EP-26 11 27, whichdescribes a time-sharing telecommunications system wherein the spectrumis exploited in a relatively efficient manner by dividing narrow-bandradio frequency channels into at least two time slots, one to transmitcoded speech signals and one to provide two-way transmission. However,this publication does not discuss the dynamic allocation of capacity,and it does not discuss packet radio services as such.

U.S. Pat. No. 4,887,265 discloses a packet radio switched cellularsystem. In this system, a plurality of data calls can be switched to oneand the same radio channel, whereby radio spectrum is saved. However,the publication mainly discusses the criteria of handoff. It does notdiscuss the dynamic allocation of capacity to packet radio and circuitswitched services.

SUMMARY OF THE INVENTION

The purpose of the invention is to indicate a method by which thecapacity of a radio channel can be better exploited. The object is amethod which is used in determining the number of time slots allocated,on the one hand, to packet radio service and, on the other hand, tocircuit switched service. According to the invention, this is done byusing the characteristics of Claim 1 by allocating dynamically morecapacity, i.e. more time slots, to the form of service requiring it at agiven time.

According to the present invention, some basic number of time slots arereserved for packet radio service and a number of time slots for circuitswitched service. If, for example, the traffic need of packet radioservice increases, information regarding this is obtained through arequest flagged by a mobile station or through traffic measurement bythe base station, whereupon this information is used as a criterion inthe allocation of more time slots to this service. Such a request can betransmitted from a mobile station to a base station on a message orsignaling channel (e.g. in call set-up signaling) or on a trafficchannel, or it may be, for example, a short message output. The requestmay also be transmitted on a packet radio channel. The capacity requiredby each service is monitored in the base station system (BSS), which isin a known manner made up of base transceiver stations (BTS) and basestation controllers (BSC), and from which information is transmitted tothe mobile station regarding the channel configurations, i.e. theallocation of radio capacity (which channel is in the use of whichservice).

In the basic mode, all the time slots of the channel may be allocated topacket radio service when circuit switched service does not needcapacity. In this case, one or several time slots are allocated tocircuit switched service when a request for it is received, for example,from a mobile station in call set-up signaling. The base stationcontroller (BSC) interprets the request from call set-up signaling andthereupon deallocates a channel which is in the use of packet radioservice and allocates it to circuit switched service. Alternatively, apredetermined minimum number of time slots may be allocated in the basicmode to packet radio service, in which case more time slots will beallocated to packet radio traffic when it increases. When the number oftime slots used for packet radio service is increased, the same numberof time slots must be deallocated from circuit switched service in thesame channel.

In a preferred embodiment of the invention, a certain minimum number oftime slots are allocated in the basic mode to packet radio service, thisminimum number being controlled on the basis of economic factorsimportant for the traffic and/or the operator. The control can becoupled to the results of traffic measurement at the base transceiverstation so that the control will be automatic. Alternatively, theoperator may effect the control manually, and it may be based onhistorical information on traffic, etc.

In sparsely populated areas, or when the packet radio traffic of a TDMAcell is known to be very scanty or incidental, all of the time slots ofthe channel serving this cell may be allocated to circuit switchedservice. When an incidental need for packet radio traffic arises,information regarding this is transmitted to the base transceiverstation, for example via the message channel of circuit switched serviceor via some other signaling channel, traffic channel or short-messageoutput. For the purpose, also a specific message may be defined whichthe mobile station requesting the service will transmit to the basetransceiver station. Preferably, however, existing messages are used forthe request, for example a part of a call set-up message minimallymodified, in which case also old services will function.

According to a preferred embodiment, more time slots for a service maybe allocated in the same frequency band so that the maximum number oftime slots allocated to the service will be equal to the number of timeslots in the band. Alternatively, time slots may be additionallyallocated to a service also in another channel.

The object is thus a method which is used in determining the number oftime slots in use, on the one hand, for packet radio service and, on theother hand, for circuit switched service. The same mechanism can be usedin parallel for FDMA bands and, on the other hand, also for only onefrequency band. In downlink packet radio control channels, the radiochannels allocated to packet switched use must be flagged very often inorder to keep the mobile stations informed of the channels currently inforce.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail with the help of thefollowing examples, with reference to the accompanying figures wherein:

FIG. 1 shows time slots in a TDMA frame; and

FIG. 2 is a block diagram of a TDMA mobile communication system inaccordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

In packet radio cellular systems, such as the GPRS system (GeneralPacket Radio Service) standardizable for GSM/DCS 1800 systems, some ofthe channels must be allocated to packet radio traffic. In this case theeasiest method is that one time slot or multiple time slots is/areallocated permanently to packet radio traffic, and the rest of the timeslots are reserved for circuit switched services. However, it ispossible to divide the capacity even more flexibly between circuitswitched service and packet radio service. The TDMA frame of one carrierwave of the GSM/DCS 1800 system is illustrated in FIG. 1. The upper rowshows the TDMA frame MS RX received by a mobile station and the lowerrow the transmission frame MS TX, offset with respect to time. Thefigure shows how one time slot in both downlink direction and uplinkdirection is reserved for packet radio service GPRS. The rest of thetime slots are reserved for circuit switched (CS) calls.

Packet radio service in TDMA systems is quite new as compared withmethods dividing a common frequency band, such as Aloha-based protocol.In TDMA radio systems using packet transmission, usually a plurality ofusers share one carrier wave time slot (frequency band) in both uplinkand downlink directions. As in GSM/DCS 1800 systems, one time slot outof eight time slots of 200 kHz radio carrier wave could be allocated tothe GRPS channel. About 25 kbit/s could be transferred in this selectedtime slot.

The capacity of one time slot may be insufficient. In this case theoperator could allocate also another time slot, and even all time slotsof a 200 kHz radio carrier wave, to packet radio service.

When multiple time slots are available, their use can be either combinedor separate. In combined use, the time slots of the same carrier waveform for transmission a capacity pool shared by the mobile stations. Oneof the same mobile stations may use multiple time slots in order toachieve a higher data rate for packet transmission. In separate use,each time slot is a separate physical packet transmission channel, andsuch channels are allocated to the mobile stations in a cell so as toachieve an even distribution of traffic. The distribution may becontrolled, for example, in the same manner as mobile stations in a GSMsystem are divided into separate paging groups.

In both cases, in time slot sharing and in separate use, increasingpacket radio traffic in the cell may be served better by allocating,when need increases, more item slots to packet radio service.

FIG. 2 is a block diagram of a time division multiple access (TDMA)mobile system 10 which comprises a base station system (BSS) 12comprising at least one base transceiver station (BTS) 14 and basestation controller (BSC) 16, as well as mobile stations 18. Two-waytraffic between the base transceiver stations 14 and mobile stations 18takes place in time slots on predetermined channels, and in which mobilesystem there is a packet radio service and a circuit switched service.In accordance with this invention, there is provided a dynamic divisionof radio capacity between packet radio service and circuit switchedservice.

In a basic mode the base station system (BSS) 12 reserves for packetradio service a predetermined first number of time slots and for circuitswitched service a predetermined second number of time slots. The basestation system 12 monitors the capacity required for each service, andallocates to packet radio service and/or circuit switched service, onthe basis of a predetermined criterion, one or more additional free timeslot/slots, while deallocating a corresponding number of time slots fromthe other service. The base station system 12 also transmits to a mobilestation 18 information regarding the division of radio capacity betweenpacket radio service and circuit switched service.

EXAMPLE 1

A situation is examined in which circuit switched connections are not inuse. In this case, all time slots are allocated to packet radio service.The timing requirements of a packet radio channel are very strict. Whena mobile station 18 transmits a request for the transmission of apacket, the base station system 12 must reserve a time slot for thismobile station within a few hundreds of milliseconds, or even faster.For this reason there must always be channel capacity available forpacket radio service.

In a cellular network, calls are set up much more slowly. Normally acall is set up within about 3-5 seconds. When the base transceiverstation 14 detects signaling required for the setting up of amobile-originated or mobile-terminated circuit switched call, one of thepacket radio channels can be reserved for such a circuit switched call.Normally, the maximum transmission time within one and the samereservation in packet radio service is limited. It is a reasonableassumption that the transmission time is below 1 second. If there istransmission on all of the packet radio channels, or on only one ofthem, the transmission of the packet is completed before the samechannel needs to be allocated to circuit switched use. However, a numberof time slots must be reserved for packet radio service, these timeslots being always available for this service so that a certain minimumlevel of service can be guaranteed in the cell. This may be a parameteradjustable at the base transceiver station. The operator may adjust thisparameter on the basis of the amount of traffic and economiccalculations. When a circuit switched call has been completed, thechannel is again released for packet radio service.

By means of this algorithm, rapid packet transmission can beaccomplished without disturbing circuit switched traffic. The capacityfor circuit switched traffic may be increased when the number of timeslots reserved for packet radio service exceeds the minimum capacityindicated by the parameter.

The adjustment of the above-mentioned parameter could, of course, alsobe automated, for example on the basis of the traffic requirementmeasured by the base transceiver station, as described below.

In a normal operating mode, all the channels on one carrier wave (inGSM, eight time slots or channels in one burst) are reserved for packetradio services (GPRS). The signaling channels may be on a separatecarrier wave or in certain allocated time slots, and the logicalchannels on the GPRS carrier waves. Mobile stations use the conventionalsignaling of the mobile telephone system for call set-up. From callset-up signaling the base station system (BSS) 12 detects that a call isarriving in the cell. The base transceiver station detects this after amobile station 18 has transmitted to it a Channel Request, which issignaled by the base transceiver station (BTS) 14 to the base stationcontroller (BSC) 16 in a Channel Required message. The base stationcontroller 16 of the base station system 12 checks whether there arefree traffic channels, whereupon a channel in the use of packet radioservice is released for circuit switched calls as follows. If a freetraffic channel is found, the mobile station 18 is given a free channelas soon as it needs a channel. If a free channel is not found,reallocation of the packet radio channel is initiated and the channel isallocated to circuit switched use immediately when needed.

The base station system 12 needs a channel before it transmits anAssignment Request to the mobile station 18. The system will have ampletime to reallocate the channel before the transmission of the AssignmentRequest. Within the time between the Assignment Request and the ChannelRequired message, signaling takes place on the signaling channel, inwhich case the channel in the use of packet radio service can be useduntil the Channel Required message is transmitted. After the channel hasbeen taken into the use of circuit switched service and after thecircuit switched call is completed, the channel is free and isreallocated to packet radio service.

The base station system 12 keeps the mobile station 18 informed of thedivision of radio capacity, i.e. transmits to the mobile stationinformation as to which channels are in the use of which service (i.e.it transmits information regarding so-called channel configurations).

The checking of capacity and the channel deallocation, according to theinvention, between packet radio and circuit switched services may beespecially necessary in the case of handover, i.e. when a mobile stationmoves from one cell to another cell, where, for example, all channelsare in the use of packet radio service. In this case the capacitychecking and channel reallocation, described above, are preferablycarried out after the base station system 12 has received from themobile station 18 a Handover Request and before the base station system12 transmits to the mobile station 18 a Handover RequestAcknowledgement. In other respects the handover procedure is defined inthe specifications of the mobile system and is therefore known to anexpert in the art and is not described here in greater detail.

EXAMPLE 2

The following situation is examined as another alternative. A certainminimum service level is required in a cell. For example, one time slotcould always be reserved for packet radio service. The other time slotsare used for circuit switched services, or they are free. The basetransceiver stations 14 measure the quantity of traffic transmitted onthe packet radio channel. Of course, the channel allocation behavior ina cell must be examined carefully before application, for example on thebasis of traffic measurements of the cell concerned or on the basis ofhistory data obtained from corresponding cells elsewhere. In theutilization of a channel there is a certain percentage limit, and at autilization ratio higher than this the channel becomes overloaded andthe service level weakens. If the utilization ratio of a channel reachesthis value, another time slot must be reserved for traffic. This can beapplied to even eight time slots within one GSM carrier wave and even tomultiple carrier waves.

When the utilization ratio of a channel decreases and reaches another,lower, level, one of the time slots in packet radio use can bedeallocated.

The algorithm according to this second alternative gives a higherpriority to packet radio service, and circuit switched services can usethe capacity not required by packet radio services.

The determination of the threshold values may be based on long-termfollow-up of traffic. Alternatively it may be variable in such a mannerthat by using the measuring results obtained within a specified pasttime period the threshold values are updated regularly.

EXAMPLE 3

The following case is discussed as one further alternative. A certainservice level, which may be quite low, is always offered in a cell. Amobile station 18 may make a request to the channel control block for afew time slots, one or several. On the basis of this request the basetransceiver station 14 allocates more time slots to packet radioservice. One channel must, of course, be allocated very rapidly to themobile station 18. In this case additional channels cannot be reservedimmediately. The mobile station 18 is given the basic channel. Duringthe Assignment Request concerning the subsequent packet the network hashad enough time to allocate multiple channels to packet radio service.Thus the mobile station 18 may gain access to the additional capacity itrequested initially, i.e. multiple time slots for transmission.

EXAMPLE 4

One further alternative is discussed below. In a rural area theallocation of even one time slot, or part of a time slot, for packetradio service may be excessive if, for example, in the area there isonly one mobile station, which transmits data once a week. In this caseno basic capacity for packet radio service needs to be reserved in thecell.

When the mobile station 18 needs to transmit packet data, it transmits amessage to that effect of the network. Since now no free packet radiochannels are available, the message may be transmitted on conventionalsignaling channels which are used for circuit switched services. Themessage may also be transmitted on other signaling channels, trafficchannels or as a short-message output.

EXAMPLE 5

The above examples 1 and 2 can be used in combination so as to ensurethat both circuit switched services and packet radio services share thechannel capacity in a “fair” manner. This means that a certain channelcapacity is allocated to the use of both services according to asemi-fixed principle, and the services use other channels by using acombination of the algorithms of Examples 1 and 2 so that congestionsituations with respect to packet radio services will be under control.The base transceiver station 14 transmits via control channelsinformation to the mobile stations 18 so that they can compete forchannels, and also that the access into the network by mobile stations18 using low priority packet radio service could be prevented until morechannels are available.

The foregoing examples are not intended to limit or restrict theteaching of this invention, which is instead intended to be given ascope commensurate with the scope of the following claims.

What is claimed is:
 1. A method for dynamic division of radio capacitybetween packet radio service and circuit switched service in a TDMAsystem in which two-way traffic between base station systems and mobilestations takes place in time slots on predetermined channels,characterized in that: in a basic mode a base station system reserves afirst predetermined number of time slots for packet radio service and asecond predetermined number of time slots for circuit-switched serviceon said predetermined channels, and that on the basis of a predeterminedcriterion, said base station system allocates at least one additionaltime slot to one of the packet radio service and circuit switchedservice, said predetermined criterion being selected from the groupconsisting of: a new packet radio connection is set up; an existingpacket radio connection is terminated; a new packet radio session is setup: an existing packet radio session is terminated; a handoverconcerning a certain mobile station is performed; and a certainthreshold value is reached in a traffic measurement performed at saidbase station system.
 2. A method according to claim 1, characterized inthat said first predetermined number of time slots comprises all thedata transmission time slots of at least one of said predeterminedchannels when circuit switched service does not require capacity, suchthat no time slot is reserved for circuit switched service on said onepredetermined channel, and that at least one time slot is allocated tocircuit switched service when said predetermined criterion is fulfilled.3. A method according to claim 1, characterized in that said firstpredetermined number of time slots comprises a predetermined minimumnumber of time slots, and on the basis of said predetermined criterion,one time slot is first allocated to the packet radio service and duringthe assignment of the subsequent packet the necessary number ofadditional time slots are allocated.
 4. A method according to claim 3,characterized in that said minimum number of time slots is adjusted onthe basis of traffic and/or economic factors.
 5. A method according toclaim 1, characterized in that said predetermined criterion furthercomprises a request received from a mobile station, the requestrequesting network capacity for one of packet radio service and circuitswitched service.
 6. A method according to claim 5, characterized inthat said predetermined first number of time slots is zero, and a mobilestation, in order to have access to packet radio service capacity, makesa request for said packet radio service capacity to said base stationsystem.
 7. A method according to claim 1, characterized in that saidpredetermined criterion comprises threshold values obtained by trafficmeasuring occurring at said base station system and that, when thetraffic of the service concerned increases to above a predeterminedfirst threshold value at least one additional time slot is allocated tothis service and, when the traffic of the service concerned drops tobelow a predetermined second threshold value, a corresponding number oftime slots are deallocated from this service.
 8. The method according toclaim 7, characterized in that the service concerned is packet radioservice and that additional time slots are allocated, when necessary,from mutiple channels.
 9. A method according to claim 7, characterizedin that the determination of the threshold values is based on at leastone of long-term measurement of traffic behavior, and an adjustment ofat least one of the threshold values on the basis of the trafficmeasurement result over a specified time period.
 10. A method accordingto claim 1, characterized in that the method is applied in parallel tothe FDMA bands in said TDMA system such that said FDMA bands are treatedlike said time slots.
 11. A time division multiple access (TDMA) mobiletelecommunication system, which comprises a base station system (BSS),and mobile stations, and in which two-way traffic between the basestation system and the mobile stations takes place in time slots onpredetermined channels, and in which mobile stations there is packetradio service and circuit switched service, characterized in that, fordynamic division of radio capacity between packet radio service andcircuit switched service, said TDMA system comprises: means for causing,in the basic mode, the base station system (BSS) to reserve for packetradio service a predetermined first number of time slots, and forcircuit switched service a predetermined second number of time slots,means, in the base station system (BSS), for monitoring the capacityrequired for each service, means for setting a predetermined criterion,said predetermined criterion being selected from the group consistingof: a new packet radio connection is set up; an existing packet radioconnection is terminated; a new packet radio session is set up; anexisting packet radio session is terminated; a handover concerning acertain mobile station is performed; and a certain threshold value isreached in a traffic measurement performed at said base station system,and means for allocating to one of the packet radio service and circuitswitched service, on the basis of said predetermined criterion, at leastone additional time slot.
 12. A mobile telecommunication systemaccording to claim 11, characterized in that the base station systemcomprises means for transmiting to at least one of said mobile stationsinformation regarding the division of radio capacity between packetradio service and circuit switched service.
 13. A method according toclaim 1, characterized in that said predetermined first number of timeslots is 0, and that a mobile station in order to have access to packetradio service capacity makes a request for said access to said basestation system.
 14. A mobile telecommunication system according to claim11, characterized in that said causing means comprises: means forsetting said first predetermined number of time slots to all the datatransmission time slots of at least one of said predetermined channelswhen circuit switched service does not require capacity, such that notime slot is reserved for circuit switched service on said onepredetermined channel; and means for allocating at least one time slotto circuit switched service when said predetermined criterion isfulfilled.
 15. A mobile telecommunication system according to claim 11,characterized in that said causing means comprises: means for settingsaid first predetermined number of time slots to a predetermined minimumnumber of time slots; and means, on the basis of said predeterminedcriterion, for first allocating one time slot to the packet radioservice and during the assignment of the subsequent packet allocatingthe necessary number of additional time slots.
 16. A mobiletelecommunication system according to claim 15, characterized in thatsaid causing means comprises means for adjusting said minimum number oftime slots on the basis of traffic and/or economic factors.
 17. A mobiletelecommunication system according to claim 11, characterized in thatsaid predetermined criterion set by said means for setting apredetermined criterion comprises a request received from a mobilestation requesting network capacity for one of packet radio service andcircuit switched service.
 18. A mobile telecommunication systemaccording to claim 11, characterized in that said predeterminedcriterion set by said means for setting a predetermined criterioncomprises threshold values obtained by traffic measuring occurring atsaid base station system, and further comprising: means, when thetraffic of the service concerned increases to above a predeterminedfirst threshold value, for allocating at least one additional time slotto this service; and means, when the traffic of the service concerneddrops to below a predetermined second threshold value, for deallocatinga corresponding number of time slots from this service.
 19. A mobiletelecommunication system according to claim 18, characterized in thatthe service concerned is packet radio service and further comprisingmeans for allocating additional time slots, when necessary, frommultiple channels.
 20. A mobile telecommunication system according toclaim 11, characterized in that said means for allocating comprisesmeans for applying said mobile telecommunication system in a TDMA systemand in parallel to the FDMA bands in said TDMA system such that saidFDMA bands are treated like said time slots.